In this study samples have been collected and analysed from throughout the terrane and this article reports the results of a subset of the samples that have been analysed for radiogenic isotopes (Phillips et al. 2017).

The age of the Siletz terrane has generally been constrained using 40Ar-39Ar whole rock dating techniques, placing the maximum age range of the terrane between 56 and 49 Ma, with the majority of the terrane having erupted between 54 and 50 Ma (Brandon, 2014; Duncan, 1982; Pyle et al., 2009). In addition to this, limited U–Pb ages have also been determined. Haeussler et al. (2000) reported a U–Pb zircon age of 50.5 Ma for the Bremerton Igneous Complex in the Crescent Formation and U–Pb zircon ages of 52 and 54 Ma have been determined for the volcanics of the Metchosin Igneous Complex by Yorath et al. (1999)..

Accretion is thought to have occurred rapidly after, and probably even during, the eruption of the terrane, first in the south between 51 and 49 Ma (Wells et al., 2014) and subsequently in the north from 48 Ma to 45 Ma (McCrory and Wilson, 2013; Wells et al., 2014).

At the time of formation (Early Eocene) there were 4 (or 5) plates in the region; the Farallon, Kula, Pacific, North American (and Resurrection) plates (Fig. 2) (Haeussler et al., 2003; Seton et al., 2012). During this time a major adjustment in spreading direction occurred between the Pacific and Farallon plates from WSW–ENE to E– W (Atwater, 1989). The Pacific-Kula ridge also underwent a change in spreading direction from N–S to NW–SE (Seton et al., 2012) before an eventual cessation of spreading (Engebretson, 1985). Most early Eocene plate reconstruction models agree that the Farallon–Kula ridge was striking NE–SW at the time of formation of the Siletz terrane, intersecting the North American plate to the east, forming a triple junction with the Pacific plate in the west and a slab window at the adjacent margin of the North American continent (Atwater, 1989; Breitsprecher et al., 2003; Engebretson, 1985; Madsen et al., 2006; Seton et al., 2012).

The Kula-Resurrection ridge probably subducted by 50 Ma (Breitsprecher et al., 2003; Haeussler et al., 2003; Wells et al., 2014) and the Siletz terrane is suggested to have formed on, or adjacent to, the mid-ocean ridge located between the Resurrection plate and the Farallon plate (McCrory and Wilson, 2013). It has also been argued that at this time (and at least since 60 Ma) the Yellowstone hotspot was located offshore of the North American continent; however, its location remains unclear (McCrory and Wilson, 2013). O'Neill et al. (2005) proposed that at 55 Ma the Yellowstone hotspot was located on the Kula–Farallon or Farallon–Resurrection ridge. Alternatively, Doubrovine et al. (2012) suggested that the hotspot was located off-axis on the Farallon plate whereas Müller et al. (1993) argued that it was adjacent to the North American continent and had moved underneath it by 50 Ma.

Geochemical results

4.1. Major and trace elements

The silica content of the analysed samples varies between 39.7 and 74.6 wt.% with the majority of samples having < 50 wt.% SiO2, while the Mg# varies between 18.5 and 70.8. The Cr abundance varies between 3.4 and 576 ppm and the Ni ranges between 3.1 and 1244 ppm. The LOIs of the samples are mostly between 0.5 and 2% but extend up to ~ 6%, which, along with the petrography, indicates that varying degrees of alteration have affected the rocks of the Siletz terrane. Under such conditions many elements, in particular the large-ion lithophile elements (i.e., K2O, Ba, Rb), may become mobile (e.g., Hastie et al., 2007; Pearce, 1996).

The samples have therefore been classified using the Zr/Ti vs. Nb/Y diagram (Pearce, 1996), and it it can be seen that the majority of samples in this study are tholeiitic basalts while a small number of samples, largely from the Siletz River Volcanics, trend towards alkaline compositions (Fig. 3).

Fig. 3. Zr/Ti–Nb/Y classification diagram adapted from Pearce (1996).

In Fig. 4 representative trace elements and ratios are plotted against Zr as it is relatively immobile and incompatible over the range of observed compositions (Cann, 1970). Many of the more immobile elements such as Nb, Th and La generally show a positive correlation with Zr (Fig. 3), and so are a more robust representation of the original magmatic variations among the rocks. Other, more mobile, trace elements such as Sr display a positive correlation in most of the samples but much more scatter is evident.

Fig. 4. Bivariate plots of trace element variations with Zr (ppm) for the Siletz terrane. Also shown in grey is the Ontong Java Plateau basalts compositional field (Fitton and Godard, 2004).

On a primitive mantle normalised diagram (Fig. 5), samples from the Siletz River Volcanics and Crescent South and some samples from Roseburg show enrichment in the most incompatible elements, while the remainder of samples show generally flat patterns. Chondrite-normalised REE patterns (Fig. 5) display flat to slightly depleted HREE and middle REE patterns (Fig. 6). The LREE in the Siletz samples show flat-to-enriched patterns, with several samples from the Siletz River Volcanics displaying the most enriched LREE patterns (La/Sm up to 9.8). There is little variation from east to west in the Dy/Yb ratio of the rocks of the Siletz terrane. The highest Dy/Yb values are found in the central part of the terrane (Fig. 6) and the lowest values are seen in the Metchosin Igneous Complex, the northernmost samples of the Crescent Terrane and from the Roseburg area in the southernmost part of the terrane. Some of these low Dy/Yb samples (from the Metchosin Igneous Complex) are also LREE depleted (Fig. 6).

Fig. 5. Primitive mantle and chondrite-normalised trace element diagrams (normalising values from Sun and McDonough, 1989) of samples also analysed for isotopic ratios. For comparison the range of compositions of Ontong Java Plateau basalts is also shown (Fitton and Godard, 2004).

The measured isotope ratios for the samples used in this study have been age corrected for in-situ decay between 54.5 and 50.5 Ma (Fig. 7. The Metchosin Igneous Complex samples display the most depleted and restricted isotope ratios of the entire Siletz terrane, and this is particularly evident in Hf–Nd isotopic space (Fig. 7d). The Crescent Formation samples show more variation in isotope ratios and this variation increases southward. The more-depleted Crescent Formation samples are still generally significantly more enriched than Pacific MORB compositions (Fig. 7). Samples CM230 and CM225 have elevated 87Sr/86Sr values compared to the other samples, which may be related to seawater alteration as all other isotope values for these samples are not anomalous. In particular Nd and Hf isotope systems are relatively resistant to alteration and so are likely to represent the primary composition of the rocks (White and Patchett, 1984; Nobre Silva et al., 2010).

The samples from northern Oregon and southern Washington (the Crescent South group of the Crescent formation, and the Siletz River Volcanics) show a similar amount of isotopic variation to one another, with the majority of samples from this area falling in the main cluster of data. The samples from the Roseburg area, in the south of the terrane, show the most isotopic variation. For example, Sample 073RB, which has the highest 206Pb/204Pb ratio of 19.668 and so plots closest to the HIMU mantle component in 206Pb/204Pb diagrams (Fig. 7), also has the highest La/Sm ratio of 9.8. Sample 095RB, also from the Roseburg area of the terrane, plots closest to EM2 and the Imnaha mantle component (IM) fields in all isotopic spaces (Fig. 7). The Imnaha component is thought to be the mantle plume source of the Columbia River Flood Basalts (CRFB) and the Snake–Yellowstone volcanics (Wolff et al., 2008).

There is little correlation between major and trace elements and isotope abundances with the exception of the more trace element-depleted samples (in comparison to the other more-enriched samples) (Fig. 5) also having the most depleted isotopic ratios of the terrane (Figs. 6 and 7). Samples with lower Pb and Sr isotopic ratios and higher εHf and εNd values also have the lowest La/Sm ratios (0.9–1.3) and the most-depleted incompatible element abundances. There is also a negative correlation between Dy/Yb and Hf isotopic ratios (Fig. 6). As this trend is distinguishable in both trace elements and radiogenic isotopes it indicates that the differences observed are a reflection of the composition of the source regions rather than varying degrees of partial melting alone. The overall variation in isotopes generally increases southward across the terrane (Fig. 6).

All samples analysed have higher Pb isotopic ratios and 87Sr/86Sr values and lower 176Hf/177Hf and 143Nd/144Nd than the Pacific MORB Mantle (PMM) (Fig. 7). The Siletz terrane samples also generally overlap the Karmutsen Flood Basalt (KFB) field, which are a series of predominantly tholeiitic basalts located on Vancouver Island and form part of the accreted Jurassic aged Wrangellia oceanic plateau (Greene et al., 2009b) (Fig. 7). In Pb-Pb isotopic space the Siletz terrane shares similar isotopic characteristics to the Caribbean Plateau (Fig. 7).

Discussion

5.1. Mantle source composition

Although the samples analysed in this study span a range of compositions there is no correlation between the degree of fractionation undergone by each sample and the radiogenic isotope ratios. This indicates that the samples are unlikely to have been contaminated with lithospheric material during fractionation and are therefore likely to record the isotopic signature of the mantle source.

The positive εHf and εNd values of all the Siletz samples reflect long term incompatible element depletion of the source. In Pb-Pb, Pb-Nd and Pb-Hf isotopic space the Siletz data lie mostly between the DGM–HIMU and DGM–EM2 mixing curves (Fig. 7), while a dominance of EM2 is required in the other diagrams. The Imnaha component also lies adjacent to the DGM-EM2 mixing curve, which indicates that the 3 samples with the significant EM2 component share similar isotopic signatures to the Columbia River Flood Basalts and the Snake River– Yellowstone hotspot trace (Wolff et al., 2008). Carlson (1984) also recognised that this component was present in the source of the CRFB, but proposed that it represented subducted sediment rather than a mantle plume component. However, the trace element data rule out the possibility of subducted sediment in the analysed samples as there is no ‘arc signature’ present in the mantle-normalised diagrams (i.e., no marked negative Nb-Ta anomaly or Th spike) (Fig. 5). Therefore, the Siletz isotope data are generally distinct from that of the Imnaha component and require a different source than the Columbia River Flood Basalts and Snake River– Yellowstone hotspot track.

There is a significant EM2 signature present in the majority of the data, indicating a deep mantle derived component (Weaver, 1991; Workman et al., 2004); this EM2. The most enriched samples display higher Dy/Yb values, indicating that they have undergone smaller amounts of melting of a deeper source relative to the more depleted samples (Fig. 6). The distinctive isotopic ratios of some of the samples may indicate that the lower melt fractions are preferentially sampling isotopically distinct enriched pods or plums (Cousens, 1996) or streaks within a mantle plume (Kerr et al., 1995).

As noted above the data also extend partially towards the HIMU component in Pb-Pb, Pb-Nd and Pb-Hf isotopic space (Fig. 7), indicating that this component may be present in the source of the Siletz basalts. The involvement of a HIMU like signature is consistent with observations in other NE Pacific locations. The samples which plot closest to HIMU have a more alkaline signature (highlighted in Fig. 7c) which suggests a relationship between the HIMU component and less extensive (deeper) melting, as is also observed at the Cobb Seamounts (Chadwick et al., 2014).

Some of the samples analysed from the Crescent Formation and Metchosin Igneous Complex lie adjacent to, or at the PREMA component composition in the Pb-Pb, Pb-Hf and Pb-Sr isotopic diagrams (Fig. 7c, d, e, f). However, while Siletz samples do appear to trend towards PREMA in some isotope diagrams (Pb-Pb), the majority of the Siletz terrane data, in most isotopic spaces, do not overlap with this component (Fig. 7). Although the Siletz terrane isotopic data demonstrates some similarity to the trends observed in, for example, the Hawaiian Islands, most notably in Hf–Nd isotopic space, the Siletz terrane samples have distinctly more radiogenic Pb isotopic ratios along with lower 87Sr/86Sr (Fig. 7). Multiple components are therefore most likely required to explain the trends observed in the isotopic data (Fig. 7) i.e., a depleted component isotopically similar to DGM, along with an enriched signature that straddles between EM2 and HIMU.

5.2. Mantle source T and P conditions

We have also attempted to determine the pressure and temperature of the mantle source region of the basalts of the Siletz terrane. Due to the relatively low magnesium contents and abundant clinopyroxene fractionation, few of the samples are suitable for the calculations. The methods outlined in Lee et al. (2009), which are based on Si and Mg contents of the magma resulted in only 7 samples producing mantle temperature and pressures, while the PRIMELTS 3 modelling software of Herzberg and Asimow (2015) produced results for 30 samples, 7 of which are in agreement with the temperature and pressure values obtained using the modelling of Lee et al. (2009). The results for these 7 samples were relatively consistent across the two techniques, producing a range of initial mantle potential temperatures (Tp) between ~1400 and 1500 °C and pressures between 1.5 GPa and 2.4 GPa (i.e., melting at less than 100 km). Mid-ocean ridge basalts display Tp ranges between ~1280 and 1400 °C, while intra-plate magmas are produced at Tp above >1400 °C (Herzberg et al., 2007; Lee et al., 2009). The Tp conditions estimated for the Siletz terrane data therefore support a mantle plume related origin for the Siletz terrane. The PRIMELTS 3 modelling results also require an olivine addition of on average 39% which indicates that subsequent to melting of the source, the magma is likely to have undergone significant fractionation in a crustal magma chamber to generate the observed compositions.

The results of this modelling, in combination with the isotopic data discussed in the previous section, indicate that the melts forming the Siletz terrane were derived from a heterogeneous and partially enriched mantle source, with an elevated temperature. The volume of melt produced (~ 2.6 × 106 km3; Trehu et al., (1994)) is consistent with extensive melting in a mantle plume head (Fitton and Godard, 2004; Herzberg and Gazel, 2009). While the range in calculated pressures may indicate inaccuracies in the models they may also point to melting over a significant range of depths which may represent the interaction of a plume and depleted mantle (Fig. 7). The amount of melting modelled in PRIMELTS3 is also indicative of a hotter-than-ambient mantle source with melting percentages of ~27% (with the majority of samples producing results between 25 and 33%), which are comparable to the extent of melting calculated for the Ontong Java Plateau and the Caribbean Plateau (~30% melting) (Fitton and Godard, 2004; Herzberg and Gazel, 2009).

5.3. Tectonic implications

There have been many tectonic models proposed to explain the origin of the Siletz terrane and its resulting geochemical and physical characteristics. One model which may explain the characteristics of the Siletz terrane is its formation in a marginal basin setting (Brandon, 2014; Wells et al., 1984), yet this does not accurately explain the proposed geometry of the plates at the time of eruption (Wells et al., 2014), the enriched isotopic signatures, nor the lack of arc signatures in the Siletz terrane (Fig. 5).

Another suggested model of formation for the Siletz terrane proposes subduction initiation along a transform fault, which can explain both the linear geometry of the terrane and its suggested volume (Stern, 2004). In this model a transform fault parallel to the North American margin between the Farallon and the partially subducted Kula/Resurrection plate represents a zone of weakness along which subduction can propagate (Babcock et al., 1992; Denny, 2012; Haeussler et al., 2003; Stern, 2004; Wells et al., 1998). The composition of the Siletz basalts does not support an early-arc tectonic setting resulting from subduction initiation as the primary mechanism of formation of the Siletz terrane. Isotopically, the Siletz basalts are much more enriched than those related to subduction initiation, e.g., in the basalts of the Izu Bonin and Mariana areas (Arculus et al., 2015; Stern, 2004).

An alternative slab window model (Babcock et al., 1992) is also not supported by the initial temperatures calculated for the mantle source of the Siletz terrane, as the temperature of primary magmas derived from slab windows are thought to be comparable to mid-ocean ridges (1300–1400 °C) (Hastie and Kerr, 2010), compared to 1400–1500 °C calculated for the Siletz terrane. The suggested characteristics of the mantle source of the Siletz terrane are also more enriched than the DMM-like mantle that is thought to produce slab window related magmatism (Hastie and Kerr, 2010; Herzberg et al., 2007). Finally, the trace elements of Siletz rocks do not generally display typical subduction related characteristics, such as Nb-Ta depletions or Th enrichments (Figs. 3 and 4) (Pearce, 2008).

The lack of uniform resemblance to the Imnaha component in the majority of Siletz samples appears to rule out the Yellowstone hotspot mantle plume head as a source region (Pyle et al., 2009). However, the composition of the plume source is unlikely to have been consistent over time, or may be heterogeneous, and therefore variation in the amount of melting for example may result in the sampling of parts of the plume with markedly different compositions (Escuder-Viruete et al., 2007; Kerr et al., 1995). In addition to this, several plate reconstructions propose that the Yellowstone hotspot was present in this area at the time of the formation of the Siletz terrane (Doubrovine et al., 2012; Müller et al., 1993; O'Neill et al., 2005).

Although volcanic ash layers/tuffs are typically unusual in oceanic plateaus (Kerr, 2014), there are abundant volcanoclastic horizons throughout the terrane, especially in the upper subaerial flows. Progressive shallowing throughout the sequence from deep to shallow water to subaerial environments indicates continual dynamic uplift by the plume (as well as accumulation of the lava pile) while also explaining the abundance of ash and volcanoclastic material. This has also been observed at other well-characterised mantle plume-related oceanic plateaus, such as in the Western Cordillera of Colombia, in the Caribbean Plateau (Kerr, 2014) and also in limited occurrences in the Ontong Java Plateau where tuff layers and sedimentary reworking occur (Mahoney et al., 2001).

The Siletz basalts are most depleted in the north and northeast, becoming more enriched in the central and southern areas and towards the west. The basalts are represented by a range of moderately enriched compositions (E-MORB-like) with even the most depleted samples being more enriched than typical N-MORB. The depth of melting also appears to increase towards the centre of the terrane. Sample 073RB is the most western sample and additionally has the largest HIMU component.

Overall, the mantle characteristics of the Siletz terrane best represent mixing of depleted mantle with an additional enriched input from a mantle plume. The temperature of the primary magmas has been calculated at 1400–1500 °C, which is hotter than ambient mid-ocean-ridge related mantle. The enriched component and mantle plume source has been proposed to represent the Yellowstone hotspot (Fig. 8) (cf. Duncan, 1982; McCrory and Wilson, 2013; Wells et al., 2014). The depleted mantle source component recorded in the relatively depleted samples from the NE (the Metchosin Igneous Complex and Crescent Formation) is comparable to mid-ocean ridge source mantle (more extensive melting), most likely the Farallon–Kula/Resurrection ridge, and the off axis interaction of an additional hotter enriched mantle source region (Fig. 8). Alternatively, the depleted component may be sampling a depleted relatively refractory portion of the mantle plume (Kempton et al., 2000; Kerr et al., 1995).

Fig. 8. Schematic diagram of the tectonic setting in which the rocks of the Siletz terrane were formed.

5.4. Youngest oceanic plateau?

Oceanic plateaus represent vast areas of over-thickened predominantly basaltic oceanic crust (>5 × 105 km3) the majority of which erupted over a few million years (Kerr, 2014; Ernst, 2014). Elevated topography and greater crustal thickness in comparison with ‘normal’ oceanic crust lead to an increase in buoyancy in oceanic plateaus. Therefore, notably for plateaus that collide with subduction zones shortly after formation (<5 Ma), the probability of partial accretion to the adjacent upper continental plate margin and so preservation within the geologic record is greatly increased (Cloos, 1993). Accreted oceanic plateaus have had a significant role in the growth of the continental crust; however, secondary processes such as deformation and burial lead to difficulties in the recognition of their characteristics once accreted (Kerr et al., 2000). Despite this, many examples of crustal growth through oceanic plateau accretion have been recognised in the geological record, including sections of the Wrangellia terrane (Greene et al., 2009a, 2009b), which along with the Siletz terrane highlights the role of oceanic plateaus in the growth and evolution of the western margin of North America.

Several well-characterised oceanic plateaus have similar heterogeneous trace element compositions to the Siletz terrane, e.g., the Cretaceous Caribbean plateau (Kerr and Mahoney, 2007) and the Late Triassic (ca. 225–231 Ma) Karmusten flood basalts of Vancouver Island and Alaska (Greene et al., 2009a, 2009b). In contrast, the largest oceanic plateau, the Cretaceous Ontong Java Plateau, is more depleted and anomalously homogeneous in composition (Fitton and Godard, 2004) (Figs. 4 and 6). While geochemistry alone cannot be used to distinguish oceanic plateaus in the geological record, their Nb/La ratio is often ~1 (Kerr, 2014), and this is generally reflected in the Siletz terrane data, which clusters between ratios of 1 and 1.5. Samples also show similar flat-to-LREE-enriched patterns with no significant Nb-Ta depletions or Th enrichments (Fig. 5).

Features of the terrane which support an oceanic plateau origin include; moderately enriched isotopic compositions, flat to slightly enriched REE patterns and the vast size and magmatic volume of the terrane (estimated to be 2.6 × 106 km3; Trehu et al., 1994), the bulk of which erupted within 5–7 Ma. In addition to this, mantle melt modelling for the Siletz terrane (as discussed above) indicates elevated mantle source potential temperatures, and similar amounts of partial melting and subsequent fractionation to the Caribbean and Ontong Java Plateaus. The terrane does, however, thin to 10 km at Vancouver Island and 6 km offshore, which is more typical of normal oceanic crust, compared to the average ~20 km thickness estimated for the remainder of the terrane (Hyndman et al., 1990; Parsons et al., 1999; Trehu et al., 1994). Sheeted dykes present at Bremerton (Crescent Formation) and on Vancouver Island (Metchosin Igneous Complex), which while unusual in other oceanic plateaus (Kerr, 2014), are consistent with plume–ridge interaction. Finally, as discussed above, there are relatively depleted signatures present in some sequences from the north of the terrane, comparable to mid-oceanic ridge environments. Overall, the accumulated evidence appears to support the conclusion that the Siletz terrane is an accreted oceanic plateau, and hence represents the youngest oceanic plateau thus far characterised.

Conclusions

The Siletz accreted mafic terrane has a large magmatic volume (2.6 × 106 km3) (Trehu et al., 1994) which represents a sequence of rocks that progressively shallow from marine to terrestrial environments and erupted over a relatively short time period (56–49 Ma).

The rocks of the terrane are geochemically comparable, both in trace element (generally flat to LREE enriched REE patterns) and radiogenic isotope composition, to several well characterised oceanic plateaus.

The estimated initial mantle potential temperatures of 1400–1500°C and the amount of partial melting undergone, ~ 25–33%, is similar to other mantle plume derived provinces such as the Ontong Java Plateau and the Caribbean plateau (Fitton and Godard, 2004; Herzberg and Gazel, 2009).

The mafic rocks of the Siletz terrane appear to have been derived from a heterogeneous and partially enriched mantle source with an above ambient temperature. This source composition is comprised of a relatively depleted signature and EM2and HIMU-like enrichments. The enriched components may therefore represent melting of a heterogeneous mantle plume, possibly the Yellowstone Hotspot, which likely interacted with a mid-oceanic ridge, the Kula–Farallon (or Farallon–Resurrection) ridge.

Although individually, the geochemical signatures and physical characteristics of the Siletz terrane can be interpreted differently, when taken together, the evidence for the Siletz terrane representing an accreted oceanic plateau linked to a mantle plume is compelling.

Atwater, T., 1989, Plate tectonic history of the northeast Pacific and western North America: The eastern Pacific Ocean and Hawaii: Boulder, Colorado, Geological Society of America, Geology of North America, N, p.21-72.

McCrory, P. A., and Wilson, D. S., 2013, A kinematic model for the formation of the Siletz-Crescent forearc terrane by capture of coherent fragments of the Farallon and Resurrection plates: Tectonics, 32, 3, 718-736.